1. Field of the Invention
This invention relates to a gas sensor, and more particularly to a gas sensor having a high resistivity against lead.
2. Description of the Prior Art
It has been known to detect the presence and concentration of a gas in air by a gas sensor which uses a gas-sensitive element made of an oxide semiconductor, such as tin oxide (SnO.sub.2), zinc oxide (ZnO), titania (TiO.sub.2), cobalt oxide (CoO), and the like, whose electric resistance varies upon exposure to the air. To simplify the structure of such gas-sensitive element for improving the productivity thereof, the so-called hybrid techniques has been developed; for instance, by printing both the gas-sensitivity element and electrodes therefor in the form of thick films on an electrically insulating ceramic substrate. It is also known that the characteristics, especially the temperature characteristics, of such gas-sensitive element can be improved by adding a platinum group element such as platinum black therein.
When the gas sensor of the prior art is used, for instance, for the detection of air fuel ratio in internal combustion engines using leaded gasoline, the gas-sensitive element of the sensor is apt to be deteriorated by lead contained in the engine exhaust gas. Such deterioration causes a shortcoming of the gas sensor in that satisfactory gas sensitivity cannot be ensured.